1. Field of the Invention
The present invention relates to a wet friction plate assembly and, more particularly, relates to an improved wet friction plate assembly to transmit torque used for a wet multiple-plate clutch, a wet multiple-plate brake and the like.
2. Description of the Related Art
A wet multiple-plate clutch and a wet multiple-plate brake (hereinafter referred to as a wet multiple-plate clutch, etc.) have, generally speaking, been used widely in an automatic transmission. The wet multiple-plate, clutch etc., has plural friction plates and separation plates which are disposed alternately and transmits torque by holding them in pressure contact hydraulically. The seizure between the friction elements on a friction plate and a separation plate is prevented by providing the lubricant in the contact surface between them, when the wet multiple-plate clutch, etc., is in disengagement (when the friction plate and the separation plate are disengaged and are not able to transmit torque).
The increase of frictional force as a result of the lubricant shear possibly induces the accompanying rotation of the separation plate, thereby causing the power loss, when an unnecessarily large amount of the lubricant remains between the friction elements of the friction plate and the contact surface of the separation plate. This phenomenon tends to occur while a wet multiple-plate clutch, etc., is in disengagement, especially at a cold start when the lubricant has high viscosity.
Japan Laid-Open Patent 11-141570 discloses a friction plate which employs plural oil passages intersecting the friction elements of the friction plate radially and discharges the lubricant remaining between the friction plate and the separation plate through the oil passages assisted by the centrifugal force.
It has been difficult to reduce the accompanying rotation induced by the lubricant dragging during the disengagement of a wet multiple-plate clutch, etc., since the sufficient discharge of the lubricant is not attained by only forming the radial oil passages between the neighboring friction elements.
Japan Laid-Open Patent 04-194422 discloses the oil passage which has a larger width inward and a smaller one outward radially. However, the reduction of the outlet of the oil passage will adversely decrease the discharge capability of the lubricant in relation to the centrifugal force.
The object of the present invention to assess the problems described above is to provide a wet friction plate assembly which is capable of discharging the lubricant existing between the friction plate and the separation plate so that the accompanying rotation of the separation plate can be restricted while a wet multiple-plate clutch, etc., is in disengagement.
The present invention in a first embodiment provides a wet friction plate assembly for a wet multiple-plate clutch, a wet multiple-plate brake and the like, comprising: a plurality of separation plates; a plurality of friction elements; and a plurality of friction plates, on contact surfaces of which the plurality of friction elements are fixed for transmitting torque to the plurality of separation plates in pressure contact, wherein the plurality of friction elements are disposed spacing at regular intervals circumferentially on each contact surface of the plurality of friction plates to form a plurality of oil passages for a lubricant between respective neighboring pairs of the plurality of friction elements, and wherein a width of each of the plurality of lubricant passages is increased progressively in a direction of inward to outward radially of each of the plurality of friction plates.
The resistance depending on the reduction of the inlet of each oil passage acts on the lubricant to be introduced into each oil passage, and the centrifugal force depending on the rotational velocity of the friction plates acts on the lubricant remaining in each oil passage, when the inlet is reduced and the outlet is expanded. If the width of the oil passage is expanded progressively from inward to outward radially, the flow velocity of the lubricant within the passage will decrease. However, the static pressure of both sides of the oil passage will also decrease and the frictional force acting on both sides of the oil passage will accordingly decrease, thereby resulting in the decrease in the lubricant pressure in a direction of discharge.
The lubricant will be discharged smoothly even if the centrifugal force acting on the lubricant to discharge it from the oil passage is small. The accompanying rotation of the separation plate induced by the friction plate will thus be restricted, compared with a straight oil passage. On the other hand, the lubricant to be introduced into an oil passage will experience the resistance depending on the reduction of the inlet of the oil passage, thereby the minimum required amount of the lubricant remains upstream the oil passage inlet, so that the seizure between the friction plate and the separation plate is prevented.
A second embodiment of the present invention provides a wet friction plate assembly for a wet multiple-plate clutch, a wet multiple-plate brake and the like, comprising: a plurality of separation plates; a plurality of friction elements; and a plurality of friction plates, on contact surfaces of which the plurality of friction elements are fixed for transmitting torque to the plurality of separation plates in pressure contact, wherein the plurality of friction elements are disposed spacing at regular intervals both radially and circumferentially on each contact surface of the plurality of friction plates to form a plurality of radial and circumferential lubricant passages between respective neighboring pairs of the plurality of friction elements, and wherein a width of each of the plurality of radial lubricant passages is increased progressively in a direction of inward to outward radially of each of the plurality of friction plates.
The multiple-stage friction elements in a radial direction provide the radial and circumferential oil passages. High pressure is generated at an intersection of each of the radial and circumferential oil passages and thereby the uniform separation of the friction plate and the separation plate can be attained. Compared with the oil passage according to the first embodiment, the length of the oil passage is shorter and thereby the frictional resistance of the lubricant will be reduced. The lubricant will also be discharged with a smaller centrifugal force because the radial oil passages communicate with the circumferential oil passage. The amount of the lubricant discharged per unit time will increase because the lubricant is discharged through both radial and circumferential oil passages. Therefore, the lubricant remaining in the oil passage can be discharged fast, which restrains the accompanying rotation of the separation plate induced by the friction plate due to the lubricant.
The lubricant to be introduced into the oil passage experiences the resistance caused by the reduction of the inlet of the oil passage, which is formed by the friction elements situated in the inner ring of the friction plate, and thereby the minimum required amount of the lubricant will remain upstream the inlet, so that the seizure of the friction plate and the separation plate is prevented.
The present invention according to a third embodiment provides a wet friction plate assembly for a wet multiple-plate clutch, a wet multiple-plate brake and the like according to the second embodiment, wherein some of the plurality of friction elements situated in an outer ring are offset circumferentially from the others of the plurality of friction elements situated in an inner ring.
The circumferential offset, which is provided for some of the friction elements situated in the outer ring and the others situated in the inner ring, makes it possible to discharge the lubricant effectively, because the Coriolis force acts on the lubricant which passes through the radial and circumferential oil passages. If the circumferential offset is determined based on the difference in the circumferential velocity between the inner and the outer of the separation plate, the accompanying rotation of the separation plate induced by the friction plate will be restricted since the lubricant is discharged more smoothly out of the radial and circumferential oil passages. It is accompanied by the prevention of the seizure of the friction and separation plates because the lubricant to be introduced into the oil passage formed by the friction elements situated in the inner ring of the friction plate experiences the resistance by the reduction of the oil passage inlet, so that the minimum required amount of the lubricant remains upstream the inlet.
The present invention according to a fourth embodiment provides a wet friction plate assembly for a wet multiple-plate clutch, a wet multiple-plate brake and the like according to the second embodiment, wherein some of the plurality of friction elements situated in an outer ring are offset a half pitch circumferentially from the others of the plurality of friction elements situated in an inner ring.
xe2x80x9cOffset a half pitchxe2x80x9d means that the friction elements are so disposed that any oil passage through which the lubricant passes from the inward to the outward of the friction plate has the same shortest distance. The friction plate with the offset improves the assembly efficiency because its directional restriction relative to the wet multiple-plate clutch, etc., is obviated. The accompanying rotation of the separation plate induced by the friction plate is restricted since the lubricant is discharged more smoothly in a direction of the Coriolis force out of the radial and circumferential oil passages. It is accompanied by the prevention of the seizure of the friction and separation plates since the lubricant to be introduced into the oil passage formed by the friction elements situated in the inner ring of the friction plate experiences the resistance by the reduction of the oil passage inlet, so that the minimum required amount of the lubricant remains upstream the inlet.